Spark igniter for combustion chambers

ABSTRACT

A spark igniter adapted to project into a combustion chamber and having a plurality of circumferentially spaced, axially extending grooves formed within its outer sleeve. The grooves are adapted to carry cooling fluid surrounding the sleeve over the end of the igniter exposed to the hot combustion gases to cool the end.

United States Patent 1 Walker et al.

[ 51 June 5, 1973 |54l SPARK IGNITER FOR COMBUSTION CHAMBERS [75] Inventors: Robert C. Walker, Glastonbury, Conn.; Sidney S. Wyde, Monticello, NY.

[73] Assignee: United Aircraft Corporation, East Hartford, Conn.

[22] Filed: Apr. 7, 1972 [21] App]. No.: 242,046

[52] US. Cl. ..60/39.67, 60/39.82 S, 317/96 [51] Int. Cl. ..F02c 7/18, F02c 7/26 [58] Field of Search ..60/39.82 S, 39.82 R, 60/39.74, 39.66, 39.82 P, 39.82 N, 39.67; 317/96-98; 123/169 C [56] References Cited UNITED STATES PATENTS 3,185,896 5/1965 Gorske et al. ..3l7/96 LIZ H fl .f 4 f5 a; Z

Primary Examiner-Carlton R. Croyle Assistant ExaminerRobert E. Garrett Attorney-Charles A. Warren [57] ABSTRACT A spark igniter adapted to project into a combustion chamber and having a plurality of circumferentially spaced, axially extending grooves formed within its outer sleeve. The grooves are adapted to carry cooling fluid surrounding the sleeve over the end of the igniter exposed to the hot combustion gases to cool the end.

11 Claims, 3 Drawing Figures I SPARK IGNITER FOR COMBUSTION CHAMBERS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to an improved spark igniter assembly particularly adapted to high temperature applications.

2. Description of the Prior Art Spark igniters for combustion chambers in gas tur bine engines have been designed with elaborate cooling schemes to prolong their life. A typical spark igniter is basically cylindrical in shape and comprises an electrode surrounded by a ceramic insulating core which is in turn surrounded by a ground shell; the operative end of the igniter projects into the combustion chamber; an electrical charge is sent through the electrode and jumps across a gap between the electrode and the ground shell creating a spark which ignites fuel within the combustion chamber.

Most of these igniter cooling schemes are keyed to reducing erosion of the ground shell in the area where the spark jumps to the shell. This problem is basically a problem whose solution involves cooling the end of the igniter projecting into the hot gases of the combustion chamber. The prior art shows that in many cases the cooling air is admitted to an annular space between the ground shell and the insulator by means of holes in the ground shell and is then ducted to the end of the igniter whereupon it convectively cools the end and exits into the combustion chamber. This technique is hereinafter referred to as the conventional manner of cooling an igniter end. The patent to Barrelle et al., U.S. Pat. No. 3,048,015, discloses such a technique.

Due to the increase in combustion chamber temperatures in the most recent gas turbine engines and in engines presently being developed for use in the near future, the life expectancy of these conventional igniters has been reduced to an undesirable and often unacceptable low level. The basic reason for this reduced life is the accelerated erosion of the ground shell due to inadequate cooling of the tip by the conventional manner under these higher temperature conditions.

An additional problem created by these higher temperatures and by the foregoing cooling technique is the exposure of the outer surface of the insulating core to cool air, creating an extremely high thermal radial gradient across the core. This high gradient often causes premature and unpredictable cracking of the insulating core. A further complication is the use of water injection in the diffuser section of many of the newer gas turbine engines. A portion of the igniter projecting outside the combustion chamber is exposed to this water, and any holes in the ground shell, for admitting cooling air to the inside of the igniter, may expose the hot ceramic insulator to contact with water droplets which may have a disastrous effect due to thermal shock. For the foregoing reasons the prior art cooling techniques have proved unsuccessful for these higher temperature applications.

SUMMARY OF THE INVENTION An object of the present invention is a spark igniter having a long life in an extremely hot environment.

According to the present invention a spark igniter is provided including an outer tubular sleeve surrounding an electrode within an insulating core, the outer surface of the sleeve having a plurality of circumferentially spaced axially extending grooves therein, adapted to lead cooling fluid over the operative end of the spark igniter to cool the end.

More particularly the operative end of the igniter projects into the combustion chamber through a hole in the wall thereof. The grooves extend through the end of the igniter forming slots spaced around the periphery of the igniter end. A portion of each groove extends outside the combustion chamber and leads cooling fluid from outside the combustion chamber through the slots in the igniter end whereupon the cooling air flows into the combustion chamber and forms a protective film of cool air over the igniter end.

In the particular embodiment hereinafter described, the tubular sleeve also acts as a ground shell, and its end, projecting into the combustion chamber, is continuously being eroded by the spark jumping from the electrode to the end of the ground shell making it imperative to cool the end of the igniter efficiently to prolong its life. In this regard a particular advantage of this invention is that even if the outer surface of the sleeve contacts the wall of the combustion chamber during engine operation the flow of cooling fluid over the end of the igniter will not be materially affected. If, for example, it was attempted to provide a cooling flow over the igniter end by only providing a small annular space between the sleeve and the wall of the combustion chamber, contact between the sleeve and the wall would cut off the flow of cooling air over a significant portion of the igniter end.

In further accord with the present invention, an insulating space is provided between the insulating core and the tubular sleeve. This space may be filled with an insulating material or it may be a dead air space. The purpose of the insulating space is to reduce the thermal radial gradient across the insulating core by minimizing heat transfer from the core to the sleeve. The inner surface of the core, which is adjacent the electrode, is extremely hot and it is desirable to maintain the temperature of the outer surface of the insulating core as close as possible to the temperature of the inner surface. Exposure of the outer surface to cooling fluid, as in the prior art, or to an abutting cool surface might create intolerable radial thermal gradients across the core.

An igniter in accordance with the present invention requires no holes through the outer sleeve. The outer sleeve therefore acts as a shield to prevent water droplets from coming into contact with the insulating core should said water droplets be present in the cooling fluid surrounding the igniter as would be the case in an engine utilizing water injection.

The foregoing and other objects, features and advantages of the present invention will become more apparent in the light of the following detailed description of preferred embodiments thereof as illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1' is a partial view partly in elevation and partly in section showing a combustor assembly incorporating the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Consider, as an example of one embodiment of the present invention, a gas turbine engine combustor assembly It) as shown in FIG. 1. The combustor assembly comprises a diffuser section 14, a combustion chamber 18, a fuel nozzle assembly and an igniter 22. The diffuser section 14 is annular and comprises an inner wall 24 and an outer wall 26. The combustion chamber 18 is annular, having an inner wall 28 and an outer wall 30. The inner wall 28 is outwardly spaced from the inner wall 24 of the diffuser section 14 forming an annular passageway 32 therebetween; the outer wall 30 of the combustion chamber 18 is inwardly spaced from the outer wall 26 of the diffuser section 14 forming an annular passageway 34 therebetween.

The fuel nozzle assembly 20 comprises a nozzle support structure 36 and a fuel nozzle 38. The support structure 36 is mounted on the outer wall 26 of the diffuser section 14 by suitable means such as bolts 40. The fuel nozzle 38 is positioned within the support structure 36 and the combustion chamber 18 and sprays fuel into the combustion chamber 18 whereupon it is ignited by the igniter or spark plug 22 in a conventional manner.

The support structure 36 is provided with an aperture 44; water from a suitable source (not shown) is brought into the support structure and is injected, at various times, into the diffuser section 14 through the apertures 44 to provide additional thrust to the engine.

The igniter 22 comprises a forward portion 46 and a rearward portion 48 and is mounted by suitable means on the outer wall 26 of the diffuser section 14. The forward portion 46 includes an operative end 50 which projects into the combustion chamber 18 and is exposed to extremely high temperatures (on the order of 3,600F.). The rearward portion 48 of the igniter 22 is connected to a suitable electrical source (not shown) in a conventional manner.

- An exemplary embodiment of the present invention is best described with reference to FIG. 3. The igniter 22 comprises a centrally located electrode 52, an insulating corev 54, and a sleeve 56 having an outer surface 57; in this embodiment the sleeve 56 is also the ground shell for the igniter. The insulating core 54 is made of a nonconducting material, such as ceramic, which can withstand high temperatures; the core 54 surrounds the electrode 52 and electrically insulates the electrode from the ground shell. The core 54 comprises an inner surface 58 and an outer surface 60; the inner surface 58, being in close proximity to the electrode 52, becomes fairly hot (on the order of 900F.) during engine operation. Although a material such as ceramic can withstand extremely high temperatures, a high thermal gradient across the core 54 may result in cracks within the core with a resultant loss in electrical insulating ability. For this reason it is theoretically desirable to maintain the outer surface 60 at the same temperature as the inner surface 58. Although this may be difficult if not impractical to achieve during transient operating conditions, prior art igniters have gone in the other direction by bringing cooling air over the outer surface 60 of the core 54, as hereinbefore discussed. In the igniter embodiment herein described the ground shell 56 is radially spaced from the core 54 to provide an insulating space 62 therebetween; this insulating space 62 may be fllledwith any suitable insulating material or may be a dead air space as is shown in this exemplary embodiment. In this manner the outer surface of the core 54 is insulated from the relatively cool air (on the order of 500F. under certain engine operating conditions) within the passageway 34 and from the ground shell 56, which is also at a low temperature relative to the temperature of the core 54 due to the exposure of the ground shell to the cool air within the passageway 34. This construction is particularly advantageous in applications where water injection is used, as in the instant case, wherein some of the water injected into the diffuser section travels downstream within the passageway 34 and impinges upon the ground shell 56; the ground shell acts as a shield to prevent water from coming into contact with the outer surface 60 of the core 54.

A plurality of axially extending, circumferentially spaced grooves 64 formed in the outer surface 57 of the ground shell 56 brings cool air, which surrounds the igniter forward portion 46, over the end 50 of the igniter 22 that projects into the combustion chamber 18. A pressure differential across the outer wall 30 of the combustion chamber 18 causes the cool air to flow from the passageway 34 into the combustion chamber 18 by way of the grooves 64 and also through a small clearance 66 between the combustion chamber wall 30 and the outer surface of the ground shell 56. As is shown in FIGS. 2 and 3, the grooves 64 extend through the igniter end 50 forming a plurality of slots 68 around the periphery thereof. Cooling fluid from outside the combustion chamber exits into the combustion chamber through these slots and is carried downstream (to the left in FIGS. 1-3) by currents within the combustion chamber over the end 50 of the igniter creating a protective cooling film over said end. An igniter cooled by this technique was found to have twice as long a life as an igniter cooled in the conventional manner.

The construction hereinbefore described assures a continuous and adequate flow of cooling fluid over the end 50 even if the outer surface 57 of the ground shell comes into contact with the wall 30 of the combustion chamber. As hereinbefore stated, attempts to cool the end 50 of the igniter without the use of the grooves 64 by simply bringing cooling fluid through the clearance 66 around the igniter may be unsatisfactory because contact between the igniter 22 and the wall 30 at the upstream side of the igniter results in cutting off the flow of cooling fluid over a substantial portion of the area of the end 50. Increasing the clearance 66 to prevent contact may not be acceptable because it might permit too much air to flow into the combustion chamber.

To assure that as much as possible of the surface area of the end 50 is covered by a film of cooling fluid, alternate slots 68 are cut deeper as at 70. The arrows 72 are representative of the flow of the cooling fluid as it exits from a slot 68 and indicates how the distribution of cooling fluid is improved by increasing the depth of a slot.

Although the invention has been shown and described with respect to a preferred embodiment thereof, it should be understood by those skilled in the art that various changes and omissions in the form and detail thereof may be made therein without departing from the spirit and scope of the invention.

Having thus described a typical embodiment of our invention, that which we claim as new and desire to secure by letters patent of the United States is:

l. A spark igniter having an operative end adapted to project into a combustion chamber through a wall thereof surrounded by a cooling fluid, said igniter including an electrode, an insulating core surrounding said electrode, and a tubular sleeve surrounding said core, said sleeve having an outer surface, said outer surface having a plurality of circumferentially spaced axially extending grooves therein, said grooves adapted to lead the cooling fluid from outside of the combustion chamber over said end of said igniter to cool said end.

2. The igniter according to claim 1 wherein said tubular sleeve comprises a ground shell for said igniter assembly.

3. An igniter according to claim 2 wherein said ground shell has an end adapted to project into the combustion chamber through an opening in a wall thereof, said end being the operative end of said ground shell, said grooves extending through said end forming slots therein, a portion of each groove adapted to remain outside the combustion chamber for receiving the cooling fluid surrounding the combustion chamber and directing it through said slots to form a cooling film over said end of said ground shell, thereby reducing the temperature of said end and prolonging its life.

4. The igniter according to claim 3 wherein one of said slots has a depth greater than at least one other of said slots for providing a better distribution of cooling fluid over the igniter operative end.

5. The igniter according to claim 4 wherein alternate slots have greater depth.

6. The igniter according to claim 1 wherein said sleeve is radially spaced from said insulating core for providing an insulating space therebetween to reduce the radial thermal gradient within the core.

7. The igniter according to claim 6 and wherein said insulating space is a dead air space.

8. The igniter according to claim 1 wherein said igniter is adapted to operate in an engine utilizing water injection and said tubular sleeve comprises shielding means adapted to prevent water from contacting said core.

9. The igniter according to claim 8 wherein said sleeve comprises a ground shell for said igniter.

10. The igniter according to claim 9 wherein said sleeve is radially spaced from said insulating core for providing an annular insulating space therebetween.

11. A combustor assembly for a gas turbine engine including a combustion chamber, a wall portion surrounding said combustion chamber and radially spaced therefrom to define an annular passageway therebetween adapted to carry a cooling fluid, an igniter having its operative end projecting into said combustion chamber, said igniter including an electrode, an insulating core surrounding said electrode, and a ground shell surrounding said core, said ground shell being radially spaced from said insulating core for providing an annular dead air space therebetween to reduce the radial thermal gradient within the core, said ground shell also including an end projecting into the combustion chamber, said end being the operative end of said ground shell and being in close proximity to said electrode so that during operation a spark may jump therebetween, said ground shell having an outer surface, at least a portion of said surface being within said annular passageway and including a plurality of circumferentially spaced axially extending grooves therein, said grooves extending through said shell end forming slots therein, each groove being adapted to receive cooling fluid surrounding the combustion chamber and to direct it through said slots whereupon it cools said igniter operative end. 

1. A spark igniter having an operative end adapted to project into a combustion chamber through a wall thereof surrounded by a cooling fluid, said igniter including an electrode, an insulating core surrounding said electrode, and a tubular sleeve surrounding said core, said sleeve having an outer surface, said outer surface having a plurality of circumferentially spaced axially extending grooves therein, said grooves adapted to lead the cooling fluid from outside of the combustion chamber over said end of said igniter to cool said end.
 2. The igniter according to claim 1 wherein said tubular sleeve comprises a ground shell for said igniter assembly.
 3. An igniter according to claim 2 wherein said ground shell has an end adapted to project into the combustion chamber through an opening in a wall thereof, said end being the operative end of said ground shell, said grooves extending through said end forming slots therein, a portion of each groove adapted to remain outside the combustion chamber for receiving the cooling fluid surrounding the combustion chamber and directing it through said slots to form a cooling film over said end of said ground shell, thereby reducing the temperature of said end and prolonGing its life.
 4. The igniter according to claim 3 wherein one of said slots has a depth greater than at least one other of said slots for providing a better distribution of cooling fluid over the igniter operative end.
 5. The igniter according to claim 4 wherein alternate slots have greater depth.
 6. The igniter according to claim 1 wherein said sleeve is radially spaced from said insulating core for providing an insulating space therebetween to reduce the radial thermal gradient within the core.
 7. The igniter according to claim 6 and wherein said insulating space is a dead air space.
 8. The igniter according to claim 1 wherein said igniter is adapted to operate in an engine utilizing water injection and said tubular sleeve comprises shielding means adapted to prevent water from contacting said core.
 9. The igniter according to claim 8 wherein said sleeve comprises a ground shell for said igniter.
 10. The igniter according to claim 9 wherein said sleeve is radially spaced from said insulating core for providing an annular insulating space therebetween.
 11. A combustor assembly for a gas turbine engine including a combustion chamber, a wall portion surrounding said combustion chamber and radially spaced therefrom to define an annular passageway therebetween adapted to carry a cooling fluid, an igniter having its operative end projecting into said combustion chamber, said igniter including an electrode, an insulating core surrounding said electrode, and a ground shell surrounding said core, said ground shell being radially spaced from said insulating core for providing an annular dead air space therebetween to reduce the radial thermal gradient within the core, said ground shell also including an end projecting into the combustion chamber, said end being the operative end of said ground shell and being in close proximity to said electrode so that during operation a spark may jump therebetween, said ground shell having an outer surface, at least a portion of said surface being within said annular passageway and including a plurality of circumferentially spaced axially extending grooves therein, said grooves extending through said shell end forming slots therein, each groove being adapted to receive cooling fluid surrounding the combustion chamber and to direct it through said slots whereupon it cools said igniter operative end. 